“CYBG has received a recent specific request from one of the rating agencies for certain financial information relating to its assessment of Clydesdale Bank’s short- and/or long-term deposit rating,” the bank said.

“The outcome of this assessment could be a near-term downgrade of the short- and/or long-term deposit rating or the placing of such rating on credit watch with negative implications. CYBG is expected to have a senior standalone investment grade credit rating.”

The bank went on to say that it does not expect any downgrade to have a “material impact” on its financial outlook or cost of funding, but that it “would require Clydesdale Bank to take mitigating actions in relation to its existing secured funding programmes”.

David Duffy heads the bank as it goes independent

NAB is keen to sell Clydesdale despite choppy conditions in financial markets because the British lender has been troubled by problems including payment protection insurance mis-selling.

For its part, the British management – headed by David Duffy, the chief executive poached from Allied Irish Banks last year – hope that independence from NAB means they can focus more closely on the UK market. Mr Duffy wants to grow mortgage lending by 50pc in the next five years and small business lending by 25pc.